Thermosetting resins show excellent thermal and mechanical properties, and outstanding dimensional stability due to their chemically cross-linked networks, and are irreplaceable in numerous applications such as high-performance coatings, adhesives, composites, rubbers, light-emitting diode lenses and solar cell encapsulants.1 A combination of high hardness and good flexibility is a highly desired feature for polymer materials, especially thermosets in many applications; but it is typically difficult to achieve both properties simultaneously. Improving the flexibility of thermosets by decreasing the cross-link density or introducing flexible segments often results in softer materials. In contrast, increasing the cross-link density of thermosets or introducing rigid structures into the cross-linked network to achieve high hardness is often accompanied by increasing brittleness. Fogelstrom et al. reported a potential route to achieve hardness and flexibility at the same time via the introduction of nanoparticle fillers into the resin.2 While the surface modification of the nanoparticle and complete dispersion of nanoparticle in the resin are indispensable to get the desired properties, this approach makes the process complex and increases the materials' cost.
Thermosetting resins have played an important role in industry due to their superior dimensional stability, good processability, and high formulation flexibility for tailoring the desired ultimate properties such as high modulus, strength, durability, and thermal and chemical resistance provided by high crosslink density.3 Of all polymers produced, approximately 18% are thermosets with the worldwide annual production of more than 65 million tons during the period 2010-2015.4 However, thermoset materials also have some drawbacks.
First of all, thermoset materials cannot be reshaped or reprocessed by heat or with solvent due to their covalently crosslinked networks.5 So the recycling of thermosetting materials after use is much more arduous than thermoplastics (the recycling rate of thermoplastics has increased a lot in the past 20 years but is still unsatisfactory6). Also, with the rapid development of science and technology and the continuous improvement of living standards, the replacement cycle of many products such as electronics, which contain a significant amount of thermosets has been shortened. For instance, cell phones are used for <18 months and computers are used for <3 years before being replaced.7 The difficulties of recycling thermosets and protecting electronic components during the thermoset removal process are motivating to develop recyclable thermosets. A similar situation exists for the recycling of fiber reinforced polymer composites which are made using valuable fibers and non-degradable thermosets.8 
Another drawback in the production of many thermosets is the undesirable release of volatile organic compounds (VOCs) during the curing process since volatile solvents or monomers are often used to achieve favorable properties and low viscosity especially for thermosetting resin-based coatings and composites.9 
Meanwhile, the sustainable development of materials requires us to reduce the consumption of nonrenewable resources and be able to recycle the materials after usage. So far, most thermosetting polymers are derived from fossil resources and thermoset materials cannot be reshaped or reprocessed by heat or with solvent due to their irreversible cross-linked networks, resulting in the difficult recycling of thermosets.1,5 
The dependence on non-renewable resources (petroleum-based chemicals) is another issue for thermosetting resins. Recent years have witnessed the rapid development of bio-based polymers such as polylactic acid (PLA), polyethylene (PE), polybutylene succinate (PBS), polyhydroxyalkanoates (PHA), starch, cellulose, and natural fiber reinforced composites due to the increasing concern about the depletion of fossil reserves and greenhouse gas emissions.4b,11 In comparison with the above bio-based polymers especially thermoplastics, bio-based thermosets receive much less attention due to their lower market volume and more difficult recycling.12 